MORPHOLOGICAL VARIABILITY OF PYRITE FRAMBOIDS FROM SANTA BARBARA BASIN SEDIMENTS – A QUESTION OF NATURE VS. NURTURE?

Santa Barbara Basin sediments, spanning the time period from 2004 to 1976 were examined for pyrite framboids by SEM. Framboid sizes range from 1 to 10 µm, are typically spherical, but will grow ovoid when constricted in microfossil tests. Although approximately 50% of the framboids consist of regularly arranged euhedral subcrystals that are considered the “norm” for pyrite framboids, there are significant proportions of other morphologies. Framboids may appear (1) massive-spongy with irregular crack patterns, (2) show rounded subcrystals, (3) show interpenetrating cubes, (4) show “spiky” subcrystals that appear polycrystalline, (5) show subcrystals that are assembled of oblong subunits (Lego-balls), and (6) show subcrystals that are assembled from smaller balls (10's of nm in size) and resemble a bunch of grapes. What is the implication of these different morphologies, and what is the significance of the tiny balls? The latter have been considered remains of nannobacteria by Folk, and regardless of the controversy that surrounds them, these, as well as the other morphologies require an explanation.

Observations from calcite precipitation experiments may provide a possible answer. Precipitation of calcite from solution is strongly affected by organic compounds, such as carboxylic acid, amino acids, lipids, fatty acids, and polysaccharides, in the solution. These bio-generated organic molecules can selectively initiate nucleation of a given mineral, control orientation and growth rate of crystals, influence size, morphology, and ordering of inorganic minerals, and terminate (inhibit) growth. Depending on the organic “additive” we may produce fibers, platelets, or equant crystals, as well as “unique” crystal orientations and crystal ordering. There is currently speculation that such features are so unique that they may represent mineralogical biosignatures. Future research may show that different types of pyrite framboids indeed indicate the presence of certain organic molecules in the pore waters, and thus may attain environmental significance.